1. Field of the Invention
The present invention relates generally to means for controlling pressure and temperature in a pipeline for compressible fluids, and more specifically to the installation and use of a Joule-Thomson type expansion valve at predetermined locations in such a line, in order to achieve precise control of the temperature of the gas flowing in various portions of the line. By routing all of the gas through such a Joule-Thomson valve, and regulating the flow through the valve to produce the desired pressure drop across the valve, temperature drop to a predetermined desired temperature is also achieved.
2. Description of the Related Art
The discovery of oil and gas deposits in Arctic and sub-Arctic areas of the world has led to a need for transporting such products from their source, to other areas for further shipping or distribution. In most cases, the most efficient means of transporting the oil and gas from the source to other areas, is by means of a relatively high capacity pipeline, due to constraints on shipping in such arctic areas during much of the year.
However, most such areas are subject to either continuous or discontinuous permafrost conditions, which leads to various problems with a pipeline carrying a compressible fluid (e.g., natural gas) thereacross. Typically, the gas is pumped at considerable pressure into the line, in order (1) to provide the required force to move the gas through the line, and (2) to increase the density of the gas in order to make the transport more efficient. It is well known that in accordance with physical laws governing the temperature and pressure of such a compressible fluid in a closed system, that as the pressure is increased, so does the temperature increase. Thus, if nothing is done to reduce the temperature of the gas, it is typically somewhat above the freezing point of water as it leaves the pumping or compressor station. If the pipeline is placed directly on or in close proximity to the permafrost terrain, the ice of the permafrost will be melted due to heat transfer from the relatively warm gas and pipe. This can cause the pipe to sink into the thawed ground and sag, potentially damaging the pipe.
On the other hand, the cooling of the gas to a temperature below the freezing point of water may also lead to problems. In areas where the permafrost is discontinuous, or where the upper layer of soil (known as the active layer) has thawed during the warm season, the cooling of the gas (and associated pipeline) to a temperature below freezing, can lead to ice formation below and around the pipe. A combination of cold pipe temperature, presence of ground water, and frost susceptible soils, can result in frost heaving around the pipe either raising the pipe and/or inducing stress on the pipe.
Typically, gas pipelines in continuous or discontinuous permafrost conditions have been designed to operate entirely in either the warm mode, i.e., above zero degrees Celsius, or the cold mode, i.e., below zero degrees Celsius, between compressor stations. It will be seen that this is not a completely satisfactory solution to the above described problems of thaw settlement and frost heave, as the soil conditions are likely to change between compressor stations. Also, the operation of a section of pipeline in either the entirely warm or entirely cold mode between compressor stations, results in relatively high gas and pipeline temperatures immediately downstream of the compressor for pipelines operating in the warm mode, or in quite cold gas and pipeline temperatures close to the next compressor station downstream for pipelines operating in the cold mode. This is due to the frictional pressure losses in the gas flow through the pipe, and the resulting drop in temperature of the gas downstream of each compressor station.
Accordingly, relatively high or low pipeline operating temperatures may be avoided by operating in the warm mode for a portion off its length immediately downstream of a compressor station, with the temperature transitioning to the cold mode (below freezing) at some intermediate point between compressor stations.
It will be seen that there is a need for some means of providing precise transition points between warm and cold operation of a gas pipeline at precisely predetermined locations along the length of the pipeline between compressor stations, in order to alleviate or obviate the effects of freezing and thawing on the underlying soil. The present invention meets this need by the placement of Joule-Thomson valves at predetermined points along the length of the pipeline, between compressor stations or other facilities along the line, and by adjusting the pressure drop across the valves in accordance with the pressure in the pipeline in order to achieve the desired gas and pipeline temperatures upstream and downstream of the valve. A discussion of the related art of which the present inventor is aware, and its differences and distinctions from the present invention, is provided below.
U.S. Pat. No. 2,961,840 issued on Nov. 29, 1960 to Walter A. Goldtrap, titled "Storage Of Volatile Liquids," describes the provision of a pit with refrigeration means extending thereacross. The pit is filled with water, and the refrigeration means is used to freeze a layer of ice across the pit. The water is drained, and the pit with its ice roof is used for the storage of various petroleum gases, such as butane, propane, etc. The Goldtrap storage system is not directed to the control of temperature of a moving fluid through a closed system, and provides no means of controlling differential pressures across a valve in a gas flow, as does the present invention.
U.S. Pat. No. 2,966,402 issued on Dec. 27, 1960 to Rudolph L. Hasche, titled "Treatment Of Natural Gas In Distribution Systems," describes a system for stripping heavier molecular weight gases from lighter gases at a distribution station. The gas is cooled to separate heavier molecules before any pressure drop is accomplished, with lighter gases then passed through an expansion engine to perform work and simultaneously reduce their temperature. The cooled and expanded gases are then warmed by passage through a heat exchanger, before distribution. The present invention teaches away from warming or compositional changes of gases, and serves only to cool gases and correspondingly drop the pressure across a Joule-Thomson valve at an intermediate point in a gas pipeline.
U.S. Pat. No. 3,251,191 issued on May 17, 1966 to Edwin E. Reed, titled "Frozen Earth Storage For Gas," describes a frozen earth storage system for natural gas, similar to the system described further above in the Goldtrap '840 U.S. patent. However, the Reed system is primarily directed to obviating any requirement for cooling of gases being added at ambient temperature to the system. This is accomplished by allowing the gas to vaporize from a high pressure liquid, to a cold vapor at ambient pressure. The vapor is then compressed and refrigerated using the refrigeration system for the gas reservoir. Reed does not disclose any provision of a Joule-Thomson valve in a gas transport pipeline system, nor does he teach the placement of such a valve at a predetermined position in the line in order to achieve predetermined temperatures upstream and downstream of the valve.
U.S. Pat. No. 3,298,805 issued on Jan. 17, 1987 to Herbert C. Secord et al., titled "Natural Gas For Transport," describes a method of storing natural gas for transport by ship, comprising cooling and pressurizing the gas mixture to achieve a density for compact storage during transport. Secord et al. teach away from the present invention, which is directed to the expansion of gas in a pipeline for reducing the temperature of the gas.
U.S. Pat. No. 3,733,838 issued on May 22, 1973 to Terry W. Delahunty, titled "System For Reliquifying Boil-Off Vapor From Liquefied Gas," describes two embodiments of gas cooling or refrigeration systems, wherein a cooled liquefied gas is pumped through a heat exchanger and back to a storage tank. The heat exchanger serves to cool relatively warmer vaporized gases from the storage tank, or from another source. Delahunty does not disclose the use of an expansion valve for reducing the pressure of gas in the system, and thus the temperature of the gas in the system, and controlling the expansion valve to provide a predetermined pressure, and thus temperature, decrease. Also, the present system is not adapted for use with liquefied gas.
U.S. Pat. No. 3,919,852 issued on Nov. 18, 1975 to James K. Jones, titled "Reliquefaction Of Boil Off Gas," describes a system using a refrigerant to cool the gas. Some of the boil off gas is used to drive a turbine, which is used to power the refrigeration system. In contrast, the present invention does not utilize any external refrigerant, but uses only a Joule-Thomson valve to provide the required drop in pressure and temperature.
U.S. Pat. No. 3,995,440 issued on Dec. 7, 1976 to George E. Wengen, titled "Vapor Control System," describes a system for recovering benzene vapors from a tank truck loading operation. The system uses the cooling effect of natural gas expansion at a distribution station, to cool an intermediate coolant which is then used to cool the benzene vapors. The present system is not used to cool any other fluid, but rather serves to cool the fluid or gas itself at predetermined points and to predetermined temperatures along a gas transportation pipeline, as desired.
U.S. Pat. No. 4,192,655 issued on Mar. 11, 1980 to Robert von Linde, titled "Process And Apparatus For The Conveyance Of Real Gases," describes a gas to gas heat exchanger installed with a compressor station along a gas pipeline. The system serves to cool gas exiting the compressor by using the temperature of the gas at the entrance or suction side of the compressor. The present invention does not utilize any form of intercooling between the gases at each side of a compression stage, but rather provides one or more expansion valves located separately from any compressor stations or other facilities along such a pipeline. It will be seen that the von Linde system may be disadvantageous in certain situations, as it may cool the exit gas to a lower than desired temperature, particularly when the drop in pressure and temperature between the exit of the first compressor and the next compressor in the line are considered. The present invention responds to this problem by providing pressure, and thus temperature, adjustments along the route of the pipeline between compressor stations.
U.S. Pat. No. 4,269,539 issued on May 26, 1981 to Scott W. Hopke, titled "Method For Preventing Damage To A Refrigerated Gas Pipeline Due To Excessive Frost Heaving," describes the addition of heat pipes adjacent a buried pipeline, in order to preclude the buildup of ice around the pipe and subsequent frost heaving in areas subject to intermittent thawing and freezing. The present invention responds to this problem by means of controlling the temperature of the gas flowing through the pipeline, and thus the temperature of the pipeline itself. Hopke does not utilize any expansion valve means for reducing the temperature of the gas within the pipe, as would already have to be in cold mode, i.e., below the freezing point of water, in order for the Hopke heating system to be required.
U.S. Pat. No. 4,372,332 issued on Feb. 8, 1983 to Burton T. Mast, titled "Compression Station For Arctic Gas Pipeline," describes a system in which relatively low pressure gas arriving at the station is preheated by using the heat of compressed gas from the downstream side of the compressor, somewhat like the von Linde '655 U.S. patent discussed further above. However, Mast then passes the compressed gas through a heat exchanger in order to lower the temperature of the compressed gas further. The reason for this apparatus is to avoid further pressure drops in the exit side of the line from the compressor, while still cooling the gas to the desired temperature. The present invention provides the desired temperature decrease at the desired predetermined point(s) in the line, using expansion valve(s)
U.S. Pat. No. 4,563,332 issued on Jan. 7, 1986 to Irving Weiss et al., titled "Refrigeration From Expansion Of Transmission Pipeline Gas," describes the expansion of gas to a pressure below the desired output pressure to the next stage, in order to obtain greater refrigeration from the expanded gas as its temperature is lowered. The gas is then compressed to the desired output pressure by a turbo-expander, which is operated by the expansion of the gas as its pressure is reduced at the first stage of the operation. The present invention does not reduce the gas pressure below the subsequent pressure stage at the next section of pipeline, nor is any compression stage used in the present invention, unlike the Weiss et al. apparatus.
U.S. Pat. No. 4,727,723 issued on Mar. 1, 1988 to Charles A. Durr, titled "Method For Sub-Cooling A Normally Gaseous Hydrocarbon Mixture," describes a system for liquefying a gas having various fractions of differing molecular weights, by separating the lightest weight fractions having the lowest condensation temperatures, and using those fractions as a refrigerant. The process uses conventional compression and heat exchange of the compressed gas for refrigeration. The present invention does not utilize any compression means, other than relying upon the compressor station(s) to provide flow through the expansion valve(s) of the present invention, in order to provide the desired temperature drop at the location of each expansion valve in the system.
U.S. Pat. No. 4,921,399 issued on May 1, 1990 to Lawrence E. Lew, titled "Gas Pipeline Temperature Control," describes a system using the recycle cooler of a compressor for cooling a portion of the exit gases and mixing the cooled gases with the output from the compressor, to lower the average temperature of the output gases in order to avoid thermal damage to the pipe at that point. The present system does not provide any division of the gas or partial routing of a fraction of the gas in order to accomplish the desired goal. Moreover, Lew is silent regarding any temperature adjustment at any point other than in the compressor station, while the present invention addresses the problem of temperature control at points intermediate between compressor stations.
U.S. Pat. No. 5,036,671 issued on Aug. 6, 1991 to Warren L. Nelson et al., titled "Method Of Liquefying Natural Gas," describes a system for refining natural gas, particularly for removing nitrogen gas therefrom. The system comprises compressing the gas mix to above atmospheric pressure, cooling and liquefying the pressurized gas through one or more refrigeration cycles, and expanding the gas to allow the lighter gases, such as nitrogen, to pass to the gaseous phase while the heavier hydrocarbon gases remain in the liquid phase. The present invention is not directed to the separation of any fraction of gases in the gases passing through the system, but rather to an apparatus and process for reducing the temperature to a predetermined point, of all of the gas passing through the system at some predetermined point.
U.S. Pat. No. 5,327,730 issued on Jul. 12, 1994 to Albert H. Myers et al., titled "Method And Apparatus For Liquifying Natural Gas For Fuel For Vehicles And Fuel Tank For Use Therewith," describes a system utilizing a secondary refrigerant, e.g., nitrogen gas, to cool the natural gas to the desired temperature and density for storage in a tank. The present system does not utilize any other refrigerant gases or fluid flows other than the gas which is flowing through the pipeline system with which the present invention may be used.
U.S. Pat. No. 5,372,010 issued on Dec. 13, 1994 to Gunther Gratz, titled "Method And Arrangement For The Compression Of Gas," describes a system for compressing gas for transport through a gas pipeline. Gratz compresses the gas to a pressure higher than that desired for the exit pressure from the compressor, which raises the gas to a higher temperature than desired. The hot gas is then passed through a heat exchanger, before expansion and further cooling to exit the compressor at the desired pressure and temperature. The excessive heating of the gas in the compression stage provides a greater difference between the initial gas temperature and the heat exchange medium, thus making the heat exchange operation more efficient. As in other pipeline compression systems of the prior art, Gratz is concerned with output pressure from the compressor, and teaches away from the use of a Joule-Thomson expansion valve to lower the temperature of the gas by lowering gas pressure, whereas the present invention uses the pressure drop through an expansion valve to reduce temperature.
U.S. Pat. No. 5,386,699 issued on Feb. 7, 1995 to Albert H. Myers et al., titled "Method And Apparatus For Liquifying Natural Gas For Fuel For Vehicles And Fuel Tank For Use Therewith," describes a system closely related to the system of the '730 U.S. patent to the same inventors, discussed further above. The same points of distinction between that system and the present invention, are felt to apply here.
U.S. Pat. No. 5,582,012 issued on Dec. 10, 1996 to Lev Tunkel et al., titled "Method Of Natural Gas Pressure Reduction On The City Gate Stations," describes a system wherein the incoming gas is split into two lines, with gas from one line being passed to a vortex tube and gas in the second line being passed to a conventional heating system in order to obviate excessive temperature drop. The gas from the vortex tube is split, with a fraction of that gas being passed through a heating system. The object of the Tunkel et al. system is to reduce the demands on a single heater, which would be required to heat all of the gas, and to reduce the total heating requirement for the gas. The present invention serves to cool the gas by reducing its pressure at some intermediate point in the line, and teaches away from heating the gas. Moreover, the present invention processes all of the gas passing through the system, rather than dividing the gas into two or more fractions, as is done with the Tunkel et al. system. Tunkel et al. do not use a Joule-Thomson expansion valve for the reduction of pressure in their system, as they do not desire the accompanying temperature decrease.
U.S. Pat. No. 5,778,917 issued on Jul. 14, 1998 to Ward A. Whitmore et al., titled "Natural Gas Compression Heating Process," describes a process for regulating the temperature of gas flowing through a pipeline, by providing intermediate relatively low compression stages between conventional compressor stations. The relatively low compression at the intermediate stages does not require post-compression cooling, as is generally the case with conventional systems. However, the Whitmore et al. '917 U.S. patent does not consider the need for controlling the temperature of the gas by cooling at intermediate points between compressor stations, which need is responded to by the present invention.
British Patent Publication No. 1,030,600 published on May 25, 1966 to Sulzer Brothers Ltd., titled "Improvements Relating To The Liquefaction Of Gases With Low Boiling Points," describes a process for optimizing the liquefaction of a gas such as helium. The process involves compressing the gas and then cooling the gas below its inversion temperature, i.e., to a point where the Joule-Thomson effect is positive for such a gas. Several other heat exchange, compression, and expansion steps are involved, with the end result being the liquefaction of the gas. The present invention is not directed to the liquefaction of a gas, and does not involve dividing the gas flow into two or more components, as does the Sulzer Brothers Ltd. system. The present system provides for the control of the temperature of gas flowing through a pipeline to a predetermined temperature well above the liquefaction point, using only an expansion valve and the energy of the gas flow.
British Patent Publication No. 1,596,330 published on Aug. 26, 1981 to Constructors John Brown Ltd. et al., titled "Gas Liquefaction," describes a system for liquefying natural gas for shipboard transport, particularly from an offshore site. The system involves a heat exchange process between the gas in the gaseous state and a liquefied gas (e.g., liquefied air or nitrogen), resulting in the vaporizing of the liquefied gas, which has a boiling point lower than that of methane at standard atmospheric pressure. The present system does not involve heat exchange with another gas, particularly an atmospheric gas with such a low boiling point.
Soviet Patent Publication No. 1,390,476 published on Apr. 23, 1988 provides a schematic illustration of an automated control system for a gas pipeline. No specific mechanism for controlling the characteristics of the gas flow (pressure and temperature) are apparent in the Soviet Patent Publication, as opposed to the Joule-Thomson expansion valve means used in the present invention.
Finally, German Patent Publication No. 4,223,160 published on Jan. 13, 1994 to Gunther Gratz illustrates a system for the compression of a gas in a gas pipeline. The German Patent Publication is the parent document for the '010 U.S. patent issued to the same inventor, and discussed further above. The same points of difference raised in that discussion, are seen to apply here.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.